1. Field of Invention
The present invention relates to nuclear magnetic resonance tools.
2. Discussion of Related Art
Nuclear magnetic resonance (NMR) has become a common wireline logging service and it has been used extensively to estimate properties of rocks (such as porosity, permeability, bound fluid, etc.) and fluids (for example, saturation, viscosity, oil and gas percentage, etc.). NMR measurements, in general, are accomplished by causing the magnetic moments of nuclei (“spins”) in a formation to precess about an axis. The axis about which the nuclei precess may be established by applying a strong, polarizing, static magnetic field (B0) to the sample to align the proton spins in a direction parallel to the applied field. Next, a series of radio frequency (RF) pulses are produced so that an oscillating magnetic field B1 is applied to the sample. The static B0 and oscillating B, fields should be substantially perpendicular to one another. The RF pulse sequence may generally begin with a 90 degree pulse that rotates the magnetization from B0 substantially into the transverse plane. Additional RF pulses, for example, such as 180 degree pulses, may be applied to create a series of spin echoes. One common sequence of RF pulses that may be used is the error-correcting CPMG (Carr-Purcell-Meiboom-Gill) NMR pulse sequence, as discussed for example, in U.S. Pat. No. 6,111,409 to Edwards et al., which is herein incorporated by reference. Techniques of NMR are well known in the literature, as discussed, for example, in U.S. Pat. No. 6,346,813 to Kleinberg, which is herein incorporated by reference.
Reservoir fluids are routinely sampled so that they can be analyzed (e.g., using NMR methods) for their chemical compositions and physical properties in order to facilitate reliable reservoir simulation. Such sampling can be done using probes to extract the fluids by a pressure drawdown provided the mobility (permeability/viscosity) is above the lower limit of such probes. One example of a probe that may be used to extract reservoir fluids is the Schlumberger Modular Formation Dynamics Tester (MDT). Mobility of heavy oil may be very low for reservoirs with low permeability or lower temperatures. Many techniques are being proposed to heat up the formation prior to drawdown in order to reduce the oil viscosity. However, it is uncertain whether any of these strategies will reliably reproduce samples that are identical to the native formation fluids. Furthermore, unconsolidated sand formations may present additional challenges for MDT-type sampling tools.